High-strength thin steel sheet and method of producing the same

ABSTRACT

This disclosure provides a high-strength thin steel sheet excellent in both tensile strength and elongation with small elongation anisotropy. The high-strength thin steel sheet has a specific chemical composition and a microstructure where a total area ratio of ferrite, tempered bainitic ferrite and bainitic ferrite is 40% or more and 70% or less, an area ratio of martensite is 5% or more and 30% or less, an area ratio of retained austenite is 10% or more and 35% or less, an average equivalent circular diameter of martensite and retained austenite (secondary phase) grains is 2.0 μm or less, an area ratio of secondary phase grains having an equivalent circular diameter of 2.0 μm or more is 10% or less, and an average minor axis length of secondary phase grains is 0.40 μm or less.

TECHNICAL FIELD

This disclosure relates to a high-strength thin steel sheet and particularly to a high-strength thin steel sheet having both a tensile strength (TS) of 980 MPa or more and excellent ductility. The high-strength thin steel sheet can be suitably used for, for example, automotive parts which are to be press-formed into complicated shapes. This disclosure also relates to a method of producing the high-strength thin steel sheet.

BACKGROUND

In recent years, improved fuel efficiency of vehicles has been required from the perspective of global environment protection, which has encouraged reductions in weight of automotive bodies. Additionally, improved crashworthiness of automobiles has also been required for the purpose of ensuring the safety of passengers in case of collision. In view of these requirements, the application of high-strength thin steel sheet having a TS of 980 MPa or more to automobiles is expanding.

However, in general, when strength of a steel sheet is increased, ductility of the steel sheet decreases. Therefore, it is desirable to develop a high-strength thin steel sheet that can keep high ductility even the steel sheet has been strengthened.

In response to such a demand, for example, JP 4291860 B (PTL 1) proposes a high-strength steel sheet with improved ductility and stretch flangeability, in which most of the microstructure is made into a fine tempered martensite and the volume ratio of retained austenite is suppressed to 3% or less. The high-strength steel sheet is produced by subjecting a steel sheet having a total volume ratio of martensite and retained austenite to the total volume of the microstructure of 90% or more to a specific heat treatment. In the heat treatment, the steel sheet is heated and held in a two-phase region of ferrite phase and austenite phase, cooled to Ms point or lower, and then tempered.

JP 4362319 B (PTL 2) proposes a high-strength steel sheet excellent in delayed fracture resistance. The high-strength steel sheet contains Mo and V as essential components and has a microstructure where at least one of martensite, tempered martensite and bainite is 70% or more in area ratio and retained austenite is 5% or less in area ratio.

JP 3889768 B (PTL 3) proposes a high-strength cold-rolled steel sheet excellent in coating film adhesion and ductility. The high-strength cold-rolled steel sheet has a microstructure consisting of tempered martensite, ferrite and retained austenite, and the number of Mn-Si composite oxides on the steel sheet surface and the coverage ratio of Si oxide on the steel sheet surface are suppressed within specific ranges.

CITATION LIST Patent Literature

PTL 1: JP 4291860 B

PTL 2: JP 4362319 B

PTL 3: JP 3889768 B

SUMMARY Technical Problem

However, the conventional high-strength steel sheets as described in PTLs 1 to 3 have the following problems.

The high-strength steel sheet described in PTL 1 achieves high hole expansion formability by making most of the microstructure into a fine tempered martensite phase. However, since the volume ratio of retained austenite phase is as low as 3% or less, the elongation (El) at a tensile strength of 980 MPa or more is only about 16%, rendering the ductility insufficient.

The high-strength steel sheet described in PTL 2 is disadvantageous in terms of cost since the addition of expensive Mo and V is indispensable. Furthermore, PTL 2 does not describe anything about the formability of the high-strength steel sheet. Actually, the high-strength steel sheet described in PTL 2 does not have sufficient ductility since the volume fraction of retained austenite is small.

Regarding the high-strength cold-rolled steel sheet described in PTL 3, since the volume fraction of tempered martensite phase is too large, the TS-El balance, which is the product of tensile strength (TS) and elongation (El) and is expressed as TS×El, may be insufficient.

Furthermore, none of PTLs 1 to 3 takes the dispersion of El into consideration.

Aiming at advantageously solving the aforementioned problems, the present disclosure provides a high-strength thin steel sheet excellent in both tensile strength and elongation with small elongation anisotropy, and a method of producing the high-strength thin steel sheet.

Solution to Problem

We have made intensive studies to solve the aforementioned problems. As a result, we discovered the following (1) and (2).

(1) By strictly controlling the following (i) to (iv) for a steel sheet having a specific chemical composition, it is possible to obtain a high-strength thin steel sheet having a TS of 980 MPa or more with extremely excellent ductility as compared with conventional ones and extremely small elongation anisotropy:

(i) area ratios of ferrite, tempered bainitic ferrite, bainitic ferrite, martensite, and retained austenite;

(ii) average equivalent circular diameter of martensite and retained austenite grains;

(iii) area ratio of martensite and retained austenite grains having an equivalent circular diameter of 2.0 μm or more to the total area of martensite and retained austenite; and

(iv) average minor axis length of martensite and retained austenite grains.

(2) A high-strength thin steel sheet satisfying the conditions of (1) can be produced by subjecting a steel sheet having a specific chemical composition and a microstructure where the total area ratio of bainitic ferrite and retained austenite is 70% or more to heating, soaking, cooling and temperature holding under a set of strictly controlled conditions.

We studied on the above discoveries and completed the present disclosure. Specifically, the primary features of the present disclosure are as follows.

1. A high-strength thin steel sheet comprising:

a chemical composition containing (consisting of), by mass%,

-   -   C: 0.10% or more and 0.35% or less,     -   Si: 0.5% or more and 2.0% or less,     -   Mn: 1.5% or more and 3.0% or less,     -   P: 0.001% or more and 0.050% or less,     -   S: 0.0001% or more and 0.0100% or less,     -   Al: 0.001% or more and 1.00% or less, and     -   N: 0.0005% or more and 0.0200% or less,     -   the balance being Fe and inevitable impurities, and     -   a ratio of C content in mass % to Mn content in mass%, expressed         as C/Mn, being 0.08 or more and 0.20 or less; and

a microstructure where

-   -   a total area ratio of ferrite, tempered bainitic ferrite and         bainitic ferrite is 40% or more and 70% or less,     -   an area ratio of martensite is 5% or more and 30% or less, and     -   an area ratio of retained austenite is 10% or more and 35% or         less, to a total area of the microstructure of the high-strength         thin steel sheet,     -   an average equivalent circular diameter of martensite and         retained austenite grains is 2.0 μm or less, an area ratio of         martensite and retained austenite grains having an equivalent         circular diameter of 2.0 μm or more to a total area of         martensite and retained austenite is 10% or less, and     -   an average minor axis length of martensite and retained         austenite grains is 0.40 μm or less.

2. The high-strength thin steel sheet according to 1., where

the chemical composition further contains at least one selected from the group consisting of, by mass %:

-   -   Ti: 0.005% or more and 0.100% or less,     -   Nb: 0.005% or more and 0.100% or less, and     -   V: 0.005% or more and 0.100% or less.

3. The high-strength thin steel sheet according to 1. or 2., where

the chemical composition further contains at least one selected from the group consisting of, by mass %:

-   -   Cr: 0.05% or more and 1.0% or less,     -   Ni: 0.05% or more and 0.50% or less,     -   Mo: 0.05% or more and 1.0% or less,     -   Cu: 0.005% or more and 0.500% or less, and     -   B: 0.0001% or more and 0.0100% or less.

4. The high-strength thin steel sheet according to any one of 1. to 3., where

the chemical composition further contains at least one of, by mass %:

-   -   Ca: 0.0001% or more and 0.0050% or less, and     -   REM: 0.0005% or more and 0.0050% or less.

5. The high-strength thin steel sheet according to any one of 1. to 4., further containing a zinc or zinc alloy coated layer on surface.

6. A method of producing a high-strength thin steel sheet, including

preparing a steel sheet having the chemical composition according to any one of 1. to 4. and a microstructure where a total area ratio of bainitic ferrite and retained austenite to a total area of the microstructure of the steel sheet is 70% or more,

heating the steel sheet to 700° C. at an average heating rate of 15° C./s or more,

heating the steel sheet from 700° C. to a soaking temperature of 740° C. or higher and 860° C. or lower at an average heating rate of 15° C./s or less,

holding the heated steel sheet at the soaking temperature for 60 seconds or more and 600 seconds or less,

cooling the steel sheet to a cooling end temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 50° C./s or less, and

holding the cooled steel sheet at the cooling end temperature for 30 seconds or more and 1200 seconds or less.

7. The method of producing a high-strength thin steel sheet according to 6., further including subjecting the steel sheet after being held at the cooling end temperature to zinc or zinc alloy coating treatment.

Advantageous Effect

According to the present disclosure, it is possible to stably provide a high-strength thin steel sheet having a TS of 980 MPa or more with excellent ductility and small elongation anisotropy. The high-strength thin steel sheet is extremely suitable for being used as automotive parts which are to be press-formed into complicated shapes. Additionally, by applying the structural parts produced with the high-strength thin steel sheet to automotive bodies, it is possible to further improve crashworthiness of automobiles and to improve fuel efficiency by reducing the weight of automotive bodies. Therefore, the present disclosure can make a significant contribution to industry development.

DETAILED DESCRIPTION

The following describes the present disclosure in detail. The chemical composition and microstructure of the high-strength thin steel sheet in one embodiment of the present disclosure are defined as stated above.

[Chemical Composition]

The reasons for limiting the chemical composition of the present disclosure as stated above are described first. In the following description, “%” regarding the chemical composition denotes “mass %” unless otherwise noted.

C: 0.10% or More and 0.35% or Less

C is an element that contributes to improvement in strength of a steel sheet. C improves steel strength by dissolving itself in steel or by precipitating itself as carbides. Additionally, C concentrates into austenite to stabilize retained austenite. As a result, ductility of a steel sheet is improved. Therefore, C is an important element for controlling both strength and ductility. To obtain such effect in a steel sheet having a TS of 980 MPa or more, the C content should be 0.10% or more. The C content is preferably 0.15% or more. The C content is more preferably 0.18% or more. The C content is still more preferably 0.20% or more. On the other hand, excessive addition of C may cause deterioration of ductility due to increase in strength and impair weldability. Therefore, the C content is 0.35% or less. The C content is preferably 0.30% or less. The C content is more preferably 0.28% or less. The C content is still more preferably 0.26% or less.

Si: 0.5% or More and 2.0% or Less

Si increases strength of steel through solid solution strengthening and contributes to improvement in ductility of ferrite by enhancing strain hardenability. Additionally, Si promotes concentration of C into austenite, contributing to stabilization of retained austenite. To obtain this effect, the Si content should be 0.5% or more. The Si content is preferably 1.0% or more.

The Si content is more preferably 1.2% or more. The Si content is still more preferably 1.3% or more. On the other hand, when the Si content exceeds 2.0%, not only is the effect saturated, but also surface quality of the steel sheet deteriorates. As a result, chemical convertibility and coating property may be deteriorated. Therefore, the Si content is 2.0% or less. The Si content is preferably 1.8% or less. The Si content is more preferably 1.6% or less. The Si content is still more preferably 1.5% or less.

Mn: 1.5% or More and 3.0% or Less

Mn is an austenite-stabilizing element and contributes to improvement in ductility by stabilizing retained austenite. Additionally, Mn, as a hardenability improving element, promotes formation of martensite and contributes to improvement in strength of a steel sheet. In order to obtain the steel sheet strength required in the present disclosure, the Mn content should be 1.5% or more. The Mn content is preferably 1.6% or more. The Mn content is more preferably 1.7% or more. On the other hand, when the Mn content exceeds 3.0%, martensite excessively forms and strength increases. As a result, ductility is deteriorated. Additionally, Mn has an effect of suppressing formation of retained austenite. Accordingly, when Mn is excessively present, a required amount of retained austenite cannot be obtained, leading to deterioration of ductility. Therefore, the Mn content is 3.0% or less. The Mn content is preferably 2.7% or less. The Mn content is more preferably 2.5% or less. The Mn content is still more preferably 2.3% or less.

P: 0.001% or More and 0.050% or Less

P, which is an element inevitably mixed in the steel as impurities, effectively strengthens the steel, yet deteriorates weldability. Therefore, the P content is 0.050% or less. The P content is preferably 0.030% or less. The P content is more preferably 0.020% or less. On the other hand, excessive reduction in P content increases refining cost. Therefore, the P content is 0.001% or more. The P content is preferably 0.005% or more.

S: 0.0001% or More and 0.0100% or Less

S, which is an element inevitably mixed in the steel as impurities, forms inclusions such as coarse MnS and reduces local ductility significantly. Therefore, the S content is 0.0100% or less. The S content is preferably 0.0050% or less. The S content is more preferably 0.0030% or less. On the other hand, excessive reduction in S content increases refining cost. Therefore, the S content is 0.0001% or more. The S content is preferably 0.0005% or more.

Al: 0.001% or More and 1.00% or Less

Al is an element as with Si that promotes concentration of C into austenite and contributes to stabilization of retained austenite. From the perspective of promoting formation of retained austenite, the Al content is 0.001% or more. The Al content is preferably 0.020% or more. The Al content is more preferably 0.030% or more. On the other hand, excessive addition of Al increases production cost. Therefore, the Al content is 1.00% or less. The Al content is preferably 0.60% or less. The Al content is more preferably 0.40% or less. The Al content is still more preferably 0.30% or less.

N: 0.0005% or More and 0.0200% or Less

N is an element inevitably mixed in the steel as impurities. N bonds with carbonitride-forming elements such as Al to form precipitates, contributing to improvement in strength and refinement of microstructure. To obtain this effect, the N content should be 0.0005% or more. The N content is preferably 0.0010% or more. On the other hand, when the N content exceeds 0.0200%, anti-aging property of the steel sheet deteriorates. Therefore, the N content is 0.0200% or less. The N content is preferably 0.0150% or less. The N content is more preferably 0.0100% or less.

The chemical composition of the high-strength thin steel sheet in one embodiment of the present disclosure consists of the above elements and the balance. The balance is Fe and inevitable impurities.

In another embodiment of the present disclosure, it is possible to arbitrarily contain at least one selected from the group consisting of Ti, Nb, and V with the following contents in addition to the above chemical composition.

Ti: 0.005% or More and 0.100% or Less

Ti has an effect of precipitation strengthening through formation of carbonitrides and an effect of refining crystal grains and is an element effective for strengthening steel. When adding Ti, in order to effectively exhibit the effects, the Ti content should be 0.005% or more. The Ti content is preferably 0.010% or more. On the other hand, excessive addition of Ti saturates the effects and increases production cost. Therefore, when adding Ti, the Ti content is 0.100% or less. The Ti content is preferably 0.080% or less.

Nb: 0.005% or more and 0.100% or less

As with Ti, Nb has an effect of precipitation strengthening through formation of carbonitrides and an effect of refining crystal grains and is an element effective for strengthening steel. When adding Nb, in order to effectively exhibit the effects, the Nb content should be 0.005% or more. The Ni content is preferably 0.010% or more. On the other hand, excessive addition of Nb saturates the effects and increases production cost. Therefore, when adding Nb, the Nb content is 0.100% or less. The Ni content is preferably 0.080% or less.

V: 0.005% or More and 0.100% or Less

As with Ti, V has an effect of precipitation strengthening through formation of carbonitrides and an effect of refining crystal grains and is an element effective for strengthening steel. When adding V, in order to effectively exhibit the effects, the V content should be 0.005% or more. The V content is preferably 0.010% or more. On the other hand, excessive addition of V saturates the effects and increases production cost. Therefore, when adding V, the V content is 0.100% or less. The V content is preferably 0.080% or less.

Furthermore, in another embodiment of the present disclosure, it is possible to arbitrarily contain at least one selected from the group consisting of Cr, Ni, Mo, Cu and B with the following contents in addition to the above chemical composition.

Cr: 0.05% or More and 1.0% or Less

Cr has an effect of enhancing hardenability and promoting formation of martensite and is therefore a useful steel-strengthening element. In order to effectively exhibit such an effect, when adding Cr, the Cr content should be 0.05% or more. On the other hand, when the Cr content exceeds 1.0%, ductility deteriorates because martensite excessively forms. Therefore, when adding Cr, the Cr content is 1.0% or less.

Ni: 0.05% or More and 0.50% or Less

As with Cr, Ni has an effect of enhancing hardenability and promoting formation of martensite and is therefore a useful steel-strengthening element. In order to effectively exhibit such an effect, when adding Ni, the Ni content should be 0.05% or more. On the other hand, when the Ni content exceeds 0.50%, ductility deteriorates because martensite excessively forms. Therefore, when adding Ni, the Ni content is 0.50% or less.

Mo: 0.05% or more and 1.0% or Less

As with Cr, Mo has an effect of enhancing hardenability and promoting formation of martensite and is therefore a useful steel-strengthening element. In order to effectively exhibit such an effect, when adding Mo, the Mo content should be 0.05% or more. On the other hand, when the Mo content exceeds 1.0%, ductility deteriorates because martensite excessively forms. Therefore, when adding Mo, the Mo content is 1.0% or less.

Cu: 0.005% or More and 0.500% or Less

As with Cr, Cu has an effect of enhancing hardenability and promoting formation of martensite and is therefore a useful steel-strengthening element. In order to effectively exhibit such an effect, the Cu content should be 0.005% or more when adding Cu. On the other hand, when the Cu content exceeds 0.500%, ductility deteriorates because martensite excessively forms. Therefore, when adding Cu, the Cu content is 0.500% or less.

B: 0.0001% or More and 0.0100% or Less

As with Cr, B has an effect of enhancing hardenability and promoting formation of martensite and is therefore a useful steel-strengthening element. In order to effectively exhibit such an effect, the B content should be 0.0001% or more when adding B. On the other hand, when the B content exceeds 0.0100%, ductility deteriorates because martensite excessively forms. Therefore, when adding B, the B content is 0.0100% or less.

Moreover, in another embodiment of the present disclosure, it is possible to arbitrarily contain at least one of Ca and REM (Rare Earth Metal) with the following contents in addition to the above chemical composition.

Ca: 0.0001% or More and 0.0050% or Less

Ca has an effect of controlling the form of sulfide inclusion and is effective for suppressing deterioration of local ductility. When adding Ca, in order to obtain the effects, the Ca content should be 0.0001% or more. On the other hand, when the Ca content exceeds 0.0050%, the effects are saturated. Therefore, when adding Ca, the Ca content is 0.0050% or less.

REM: 0.0005% or More and 0.0050% or less

As with Ca, REM has an effect of controlling the form of sulfide inclusion and is effective for suppressing deterioration of local ductility. When adding REM, in order to obtain the effects, the REM content should be 0.0005% or more. On the other hand, when the REM content exceeds 0.0050%, the effects are saturated. Therefore, when adding REM, the REM content is 0.0050% or less.

The chemical composition of the high-strength thin steel sheet in one embodiment of the present disclosure can be expressed as follows:

a chemical composition containing, by mass%:

C: 0.10% or more and 0.35% or less,

Si: 0.5% or more and 2.0% or less,

Mn: 1.5% or more and 3.0% or less,

P: 0.001% or more and 0.050% or less,

S: 0.0001% or more and 0.0100% or less,

Al: 0.001% or more and 1.00% or less,

N: 0.0005% or more and 0.0200% or less,

optionally, at least one selected from the group consisting of Ti: 0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% or less, and V: 0.005% or more and 0.100% or less,

optionally, at least one selected from the group consisting of Cr: 0.05% or more and 1.0% or less, Ni: 0.05% or more and 0.50% or less, Mo: 0.05% or more and 1.0% or less, Cu: 0.005% or more and 0.500% or less, and B: 0.0001% or more and 0.0100% or less, and

optionally, at least one of Ca: 0.0001% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less,

the balance being Fe and inevitable impurities.

Additionally, the chemical composition of the high-strength thin steel sheet in one embodiment of the present disclosure may also contain components other than those described above as long as the effect of the present disclosure is not impaired.

C/Mn: 0.08 or More and 0.20 or Less

Furthermore, in the present disclosure, the ratio of C content in mass % to Mn content in mass % in the above chemical composition, expressed as C/Mn, should be 0.08 or more and 0.20 or less. The reasons for the limitation are as described below.

When a steel sheet containing retained austenite is processed, a part of retained austenite in which strain has caused by the processing transforms into martensite, in other words, strain-induced transformation occurs. Because of the strain-induced transformation, the deformed part of the steel sheet hardens and strain localization is suppressed, and accordingly uniform elongation is improved. Although C and Mn are elements that contribute to stabilization of retained austenite as described above, Mn has an effect of suppressing formation of retained austenite. Therefore, in order to obtain excellent uniform elongation, the ratio of C content to Mn content in the chemical composition of the steel sheet, expressed as C/Mn, should be appropriately controlled.

When C/Mn is less than 0.08, since the C content is small and the Mn content is large, stability of retained austenite is low and formation of retained austenite is suppressed. As a result, a required amount of stable retained austenite cannot be obtained. Therefore, C/Mn is 0.08 or more. C/Mn is preferably 0.09 or more. On the other hand, when C/Mn exceeds 0.20, since the C content is large and the Mn content is small, the concentration of C in retained austenite excessively increases. As a result, martensite excessively hardens during the strain-induced martensitic transformation, leading to deterioration of formability. Therefore, C/Mn is 0.20 or less. C/Mn is preferably 0.17 or less. C/Mn is more preferably 0.15 or less.

[Microstructure]

Next, the reasons why the microstructure of the high-strength thin steel sheet in the present disclosure is limited as described above will be explained.

Total area ratio of F+TBF+BF: 40% to 70%

Ferrite (hereinafter may be abbreviated to “F”) forms during the cooling after soaking and contributes to improvement in ductility of steel. Tempered bainitic ferrite (hereinafter may be abbreviated to “TBF”) is a microstructure where bainitic ferrite in the starting microstructure, which will be described later, has been tempered. TBF has an influence on formation of retained austenite in final microstructure, and contributes to improvement in ductility of steel like ferrite. As will be described later, bainitic ferrite (hereinafter may be abbreviated to “BF”) is formed by maintaining the cooling end temperature at 350° C. or higher and 550° C. or lower. The C released during the formation of BF concentrates into austenite to stabilize retained austenite. As a result, ductility is improved.

As stated above, F, TBF and BF all contribute to improvement in ductility. In order to ensure a required ductility, the total area ratio of F+TBF+BF with respect to the total area of the microstructure of the high-strength thin steel sheet (hereinafter “total area ratio of F+TBF+BF”) should be 40% or more. The total area ratio of F+TBF+BF is preferably 45% or more. The total area ratio of F+TBF+BF is more preferably 50% or more. The total area ratio of F+TBF+BF is still more preferably 55% or more. On the other hand, when the total area ratio of F+TBF+BF exceeds 70%, it becomes difficult to ensure a TS of 980 MPa or more. Therefore, the total area ratio of F+TBF+BF is 70% or less. The total area ratio of F+TBF+BF is preferably 65% or less.

Area ratio of M: 5% to 30%

In the present disclosure, in order to ensure the strength, martensite (hereinafter may be abbreviated to “M”) is contained in the microstructure. In order to obtain a required strength, the area ratio of M with respect to the total area of the microstructure of the high-strength thin steel sheet (hereinafter “area ratio of M”) should be 5% or more. The area ratio of M is preferably 10% or more. The area ratio of M is more preferably 15% or more. On the other hand, when the area ratio of M exceeds 30%, formability cannot be ensured. Therefore, the area ratio of M is 30% or less. The area ratio of M is preferably 25% or less.

Area ratio of RA: 10% to 35%

Retained austenite (hereinafter may be abbreviated to “RA”) hardens the deformed part through strain-induced transformation. That is to say, a part of retained austenite in which strain has occurred during the deformation of the material transforms into martensite to harden the deformed part. Therefore, the presence of RA suppresses strain localization and improves uniform elongation of the steel sheet. In order to achieve high ductility while maintaining a TS of 980 MPa or more, the area ratio of RA to the total area of the microstructure of the high-strength thin steel sheet (hereinafter “area ratio of RA”) should be 10% or more. The area ratio of RA is preferably 15% or more. The area ratio of RA is more preferably 17% or more. On the other hand, when the area ratio of RA exceeds 35%, cracks are likely to occur in flange portions during press forming. Therefore, the area ratio of RA is 35% or less. The area ratio of RA is preferably 30% or less. The area ratio of RA is more preferably 25% or less.

In the present disclosure, the area ratio of each phase can be measured by the following method.

Retained Austenite

The area ratio of retained austenite can be calculated by determining the x-ray diffraction intensity of a plane of sheet thickness ¼, which is the plane at a depth of one-fourth of the sheet thickness from the steel sheet surface and is exposed by polishing the steel sheet surface to a depth of one-fourth of the sheet thickness. Specifically, the volume ratio of retained austenite is calculated from the peak intensity ratio of (200) and (211) planes of BCC phase and (200), (220) and (311) planes of FCC phase through X-ray diffraction using Co K-alpha X-ray. The microstructure of the steel sheet of the present disclosure is three-dimensionally uniform, and therefore the volume ratio can be regarded as the area ratio of RA. Measurement is carried out at three positions randomly selected on one high-strength thin steel sheet, and the average value of the obtained values is taken as the area ratio of retained austenite.

Microstructure Other than Retained Austenite

The area ratio of each microstructure other than retained austenite can be obtained by etching a cross section in the rolling direction of the high-strength thin steel sheet with nital and then observing the cross section with a scanning electron microscope (SEM). The observation position is at the center of the width direction of the steel sheet and at a depth of one-fourth of the sheet thickness. An SEM image of the observation position is photographed at a magnification of 2000 times, and the area ratio of each microstructure is calculated through image analysis of the obtained SEM image. In the image analysis, the occupied area of each microstructure present in a randomly set 50 μm×50 μm square region is determined and the area ratio thereof is calculated. The observation is carried out at five positions randomly selected on one high-strength thin steel sheet, and the average value of the obtained values is taken as the area ratio.

Each microstructure in the image analysis can be identified by the following method.

Firstly, retained austenite and martensite are observed as massive white regions having a relatively smooth surface in the SEM image. Therefore, the area ratio of martensite can be calculated by subtracting the area ratio of retained austenite measured by the above-mentioned method from the area ratio of the white region.

On the other hand, ferrite, tempered bainitic ferrite and bainitic ferrite are observed as black regions in the SEM image. Among the black regions, those in a massive form are identified as ferrite. Additionally, among the black regions, those having an elongated shape and a recognizable substructure are identified as bainitic ferrite, and those in which substructure cannot be recognized are identified as tempered bainitic ferrite. Examples of the substructure include internal structure of lath, block, packet, etc. in grains.

Note that in the present disclosure, the simple expression “area ratio” means an area ratio with respect to the total area of the microstructure of the target steel sheet. Additionally, the grain size measured in the present disclosure has a lower limit of 0.01 μm which is the measurement limit.

Average equivalent circular diameter of secondary phase: 2.0 μm or less

In the present disclosure, it is also important to control the grain size of martensite and retained austenite (hereinafter, martensite and retained austenite are simply referred to as “secondary phase”). When the average equivalent circular diameter of grain of secondary phase (hereinafter simply referred to as “average equivalent circular diameter of secondary phase”) is large, the steel sheet does not deform uniformly since localized strain occurs during the deformation of the steel sheet. Therefore, a steel sheet having a large average equivalent circular diameter of secondary phase is inferior to a steel sheet having a small average equivalent circular diameter of secondary phase in formability. Additionally, when the average equivalent circular diameter of secondary phase is large, C does not sufficiently concentrate into austenite and stability of retained austenite deteriorates. As a result, ductility of the steel sheet is deteriorated. Therefore, the average equivalent circular diameter of secondary phase is 2.0 μm or less. The average equivalent circular diameter of secondary phase is preferably 1.8 μm or less. The average equivalent circular diameter of secondary phase is more preferably 1.6 μm or less. On the other hand, the lower limit of the average equivalent circular diameter of secondary phase is not particularly limited. Although the lower the average equivalent circular diameter is the better, an average equivalent circular diameter of 0.5 μm or more or 1.0 μm or more is acceptable from the perspective of production.

Area ratio of coarse grain in secondary phase: 10% or less

When the grain of secondary phase is coarse, stress concentrates on the interface between the secondary phase and the microstructure other than the secondary phase during the deformation of the steel sheet, leading to a high possibility of occurrence of void. Since the occurrence of void deteriorates formability, it is necessary to lower the ratio of coarse grain in secondary phase in order to improve formability. Therefore, the area ratio of martensite and retained austenite grains having an equivalent circular diameter of 2.0 μm or more to the total area of secondary phase (hereinafter “area ratio of coarse grain in secondary phase”) is 10% or less. The area ratio of coarse grain in secondary phase is preferably 9% or less. The area ratio of coarse grain in secondary phase is more preferably 8% or less. The area ratio of coarse grain in secondary phase is still more preferably 7% or less. On the other hand, since the lower the area ratio of coarse grain in secondary phase is, the better, the lower limit of the area ratio is not particularly limited and may be 0%.

Average minor axis length of secondary phase: 0.40 μm or less

In order to improve ductility of the high-strength thin steel sheet, it is necessary to make the secondary phase extended in a long and narrow shape. That is to say, when the secondary phase is in a long and narrow shape, the diffusion distance is shortened, and accordingly C and Mn can be efficiently distributed into austenite during annealing. The distributed C and Mn stabilize the austenite, and accordingly local concentration of strain during deformation of the steel sheet is suppressed and ductility is improved. To obtain this effect, the average minor axis length of martensite and retained austenite grains (hereinafter “average minor axis length of secondary phase”) should be 0.40 μm or less. When the average minor axis length of secondary phase is larger than 0.40 μm, concentration of C and Mn into austenite is insufficient, rendering stability of retained austenite low. Additionally, since martensite easily forms when the average minor axis length of secondary phase is larger than 0.40 μm, a desired amount of retained austenite cannot be obtained. On the other hand, although the lower limit of the average minor axis length of secondary phase is not particularly limited, the average minor axis length is usually 0.10 μm or more or 0.15 μm or more.

The average major axis length of secondary phase is not particularly limited, yet it is preferably about 1 μm to 10 μm.

The average equivalent circular diameter of secondary phase, the area ratio of coarse grain in secondary phase and the average minor axis length of secondary phase can be obtained by etching a cross section in the rolling direction of the high-strength thin steel sheet with nital and then observing the cross section with a scanning electron microscope (SEM). The observation position is at the center of the width direction of the steel sheet and at a depth of one-fourth of the sheet thickness. An SEM image of the observation position is photographed at a magnification of 2000 times, and the obtained SEM image is subjected to image analysis. In the image analysis, occupied area and minor axis length of the secondary phase present in a randomly set 50 μm×50 μm square region are determined. The average equivalent circular diameter of secondary phase is calculated from the obtained occupied area of the secondary phase. The measurement lower limit of the minor axis length was 0.01 μm. The observation is carried out at five positions randomly selected on one high-strength thin steel sheet, and the average value of the

[Sheet Thickness]

In the present disclosure, the high-strength thin steel sheet means a high-strength steel sheet having a sheet thickness of 0.6 mm to 6.0 mm. The high-strength thin steel sheet in the present disclosure may be a hot rolled steel sheet or a cold rolled steel sheet. When the high-strength thin steel sheet is a hot rolled steel sheet, the sheet thickness is preferably 1.2 mm to 6.0 mm. When the high-strength thin steel sheet is a cold rolled steel sheet, the sheet thickness is preferably 0.6 mm to 2.6 mm. A high-strength thin steel sheet that is a hot rolled steel sheet may be referred to as “high-strength hot-rolled steel sheet” and a high-strength thin steel sheet that is a cold rolled steel sheet may be referred to as “high-strength cold-rolled steel sheet”. In a case where the high-strength thin steel sheet further includes a coated layer, the sheet thickness means the thickness of the high-strength thin steel sheet that is the base metal steel sheet excluding the coated layer.

[Zinc or Zinc Alloy Coated Layer]

The high-strength thin steel sheet of the present disclosure can further includes a zinc or zinc alloy coated layer on surface. A high-strength thin steel sheet including a zinc or zinc alloy coated layer may be referred to as “high-strength zinc or zinc alloy coated thin steel sheet”. The “zinc or zinc alloy coated layer” here includes both of a galvanized layer and a zinc alloy coated layer. Any alloy coated layer containing zinc can be used as the zinc alloy coated layer. Preferable examples of the zinc alloy coated layer include a 5% Al-Zn alloy coated layer containing 5 mass % of Al and a 55% Al-Zn alloy coated layer containing 55 mass % of Al.

The zinc or zinc alloy coated layer may be an electro zinc or zinc alloy coated layer or a hot-dip zinc or zinc alloy coated layer. However, it is preferably a hot-dip zinc or zinc alloy coated layer. A high-strength thin steel sheet including a hot-dip zinc or zinc alloy coated layer may be referred to as “high-strength hot-dip zinc or zinc alloy coated thin steel sheet”. Additionally, the hot-dip zinc or zinc alloy coated layer may be an alloyed hot-dip zinc or zinc alloy coated layer which is subjected to alloying treatment after being hot-dip zinc or zinc alloy coated. A high-strength thin steel sheet including an alloyed hot-dip zinc or zinc alloy coated layer may be referred to as “high-strength alloyed hot-dip zinc or zinc alloy coated thin steel sheet”.

The following describes a method of producing the high-strength thin steel sheet in one embodiment of the present disclosure.

The high-strength thin steel sheet of the present disclosure can be produced by subjecting a steel sheet having a specific chemical composition and a specific microstructure to heat treatment under a set of specific conditions. Hereinafter, the steel sheet to be subjected to the heat treatment may be referred to as “starting steel sheet” and the microstructure of the starting steel sheet may be referred to as “starting microstructure”.

[Starting Steel Sheet]

In the present disclosure, the steel sheet to be subjected to the heat treatment, in other words the starting steel sheet may be anything as long as it has the chemical composition and microstructure as described below. Both of hot rolled steel sheet and cold rolled steel sheet can be used as the starting steel sheet.

[Chemical Composition of Starting Steel Sheet]

The chemical composition of the starting steel sheet can be the same as the chemical composition of the high-strength thin steel sheet described above.

[Microstructure of Starting Steel Sheet]

Total Area Ratio of BF+RA: 70% or More

In the present disclosure, a steel sheet having a microstructure where the total area ratio of bainitic ferrite and retained austenite to the total area of the microstructure of the steel sheet (hereinafter “total area ratio of BF+RA”) is 70% or more is used as the starting steel sheet. The high-strength thin steel sheet of the present disclosure can be obtained by subjecting a starting steel sheet having the microstructure to the heat treatment which will be described later. The total area ratio of BF+RA is preferably 73% or more. The total area ratio of BF+RA is more preferably 75% or more. The total area ratio of BF+RA is still more preferably 78% or more. On the other hand, since the higher the total area ratio of BF+RA is, the better, the upper limit thereof is not particularly limited and may be 100%.

The area ratio with BF alone and the area ratio with RA alone are not particularly limited as long as the above condition is satisfied. However, a too small area ratio of RA is likely to result in a lumpy secondary phase in final microstructure. Therefore, the area ratio of RA to the total area of the microstructure of the starting steel sheet is preferably 1% or more. On the other hand, a too large area ratio of RA is likely to result in a coarse secondary phase in final microstructure. Therefore, the area ratio of RA to the total area of the microstructure of the starting steel sheet is preferably 30% or less.

The microstructure of the starting steel sheet may contain any other microstructure as the balance other than BF and RA. Examples of the other microstructure include ferrite and martensite. The total area ratio of the other microstructure is 30% or less, and the lower limit of the total area ratio may be 0%.

It is preferable if the starting steel sheet further contains a microstructure where lath-shaped retained austenite exists among bainitic ferrite. A starting steel sheet containing such microstructure can maintain the lath shape of the retained austenite even after the heat treatment which will be described later. As a result, the secondary phase in the eventually obtained steel sheet microstructure tends to be in a long and narrow shape. Consequently, the concentration of C into retained austenite is sufficient to stabilize the retained austenite, which further improves ductility. Additionally, since the secondary phase is finely dispersed, strain is likely to disperse and uniformly deform during deformation of the steel sheet, which further improves formability.

In the present disclosure, the method of producing the starting steel sheet is not particularly limited and can be any one as long as the starting microstructure described above is obtainable. Nevertheless, an example of the production method will be provided below.

When the starting steel sheet is a hot rolled steel sheet, the starting steel sheet can be produced by hot rolling a steel slab having the chemical composition described above. The steel slab can be obtained by smelting and casting steel whose chemical composition has been adjusted to the above range according to a conventional method. The hot rolling can be performed, for example, by heating the steel slab to a heating temperature of 1150° C. or higher and then rolling the steel slab at a finisher delivery temperature of 850° C. or higher. The rolled steel sheet can be coiled, for example, after being cooled down to a coiling temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 30° C./s or more. By performing hot rolling and coiling under the above conditions, it is possible to obtain a starting steel sheet, which is a hot-rolled steel sheet, having the starting microstructure described above.

When the starting steel sheet is a cold rolled steel sheet, the starting steel sheet can be produced by subjecting a steel slab having the chemical composition described above to hot rolling and then cold rolling and subsequently a heat treatment in which the starting microstructure is obtained. The steel slab can be produced by smelting and casting steel whose chemical composition has been adjusted to the above range according to a conventional method. The hot rolling can be performed, for example, by heating the steel slab to a heating temperature of 1150° C. or higher and then rolling the steel slab at a finisher delivery temperature of 850° C. or higher. The rolled steel sheet can be coiled, for example, after being cooled down to a coiling temperature of 600° C. or higher and 700° C. or lower at an average cooling rate of 30° C./s or more. Subsequently, the obtained hot rolled steel sheet is pickled and then cold rolled. In the pickling, for example, hydrochloric acid can be used. The rolling reduction in the cold rolling is preferably 40% or more. Furthermore, the obtained cold rolled steel sheet is subject to a heat treatment to obtain the starting microstructure. In the heat treatment, the cold rolled steel sheet is heated to a soaking temperature and then held at the soaking temperature, followed by cooling the steel sheet down to a cooling end temperature and then holding the steel sheet at the cooling end temperature. Preferable conditions for each step are as follows. Soaking temperature is Ac₃ transformation temperature or higher; soaking time is 60 seconds or more and 600 seconds or less; average cooling rate from soaking temperature to cooling end temperature is less than 50° C./s, cooling end temperature is 350° C. or higher and 550° C. or lower, and holding time is 30 seconds or more and 1200 seconds or less. By performing a heat treatment under the above conditions, it is possible to obtain a starting steel sheet, which is a cold rolled steel sheet, having the starting microstructure described above.

The Ac₃ transformation temperature here can be calculated with the following equation of Andrews et al.

Ac₃=910−203[C]^(1/2)+45[Si]−30[Mn]−20[Cu]−15[Ni]+11[Cr]+32[Mo]+104[V]+400[Ti]+460[Al]

where the parentheses in the equation represent the content of element within the parentheses in the steel sheet in mass %.

[Heat Treatment]

Next, the starting steel sheet is subjected to a heat treatment. The following steps (1) to (5) are sequentially performed in the heat treatment:

(1) heating the steel sheet to 700° C. at an average heating rate of 15° C./s or more; (2) heating the steel sheet to a soaking temperature of 740° C. or higher and 860° C. or lower at an average heating rate of 15° C./s or less, (3) holding the steel sheet at the soaking temperature for 60 seconds or more and 600 seconds or less, (4) cooling the steel sheet to a cooling end temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 50° C./s or less, and (5) holding the steel sheet at the cooling end temperature for 30 seconds or more and 1200 seconds or less.

The following explains the conditions in each step and the reasons for limiting the conditions.

Average Heating Rate Up to 700° C.: 15° C./s or More

First, the starting steel sheet is heated to 700° C. at an average heating rate of 15° C./s or more. The average heating rate here is the average heating rate during a period from the start of heating to when the temperature reaches 700° C. Hereinafter, the average heating rate is referred to as “average heating rate up to 700° C.”. When the average heating rate up to 700° C. is less than 15° C./s, ductility of the eventually obtained high-strength thin steel sheet deteriorates. This is because that retained austenite is decomposed into ferrite and cementite during the heating when the average heating rate is less than 15° C./s. As a result, cementite produces austenite and the austenite becomes lumpy, and accordingly the equivalent circular diameter of secondary phase in final microstructure increases, leading to failure of formation of fine martensite and retained austenite. Therefore, the average heating rate up to 700° C. is 15° C./s or more. The average heating rate is preferably 17° C./s or more. The average heating rate is more preferably 20° C./s or more. On the other hand, although the upper limit of the average heating rate is not particularly limited, the rate is preferably 50° C./s or less and more preferably 45° C./s or less from the perspective of equipment and the like.

Average Heating Rate Up to Soaking Temperature: 15° C./s or Less

Then, the steel sheet is heated to a soaking temperature which will be described later at an average heating rate of 15° C./s or less. The average heating rate here is the average heating rate during a period from when the temperature is 700° C. to when the temperature reaches the soaking temperature. Hereinafter, the average heating rate is referred to as “average heating rate up to soaking temperature”. When the average heating rate up to soaking temperature is more than 15° C./s , formability of the eventually obtained high-strength thin steel sheet deteriorates. The reason is that when the average heating rate is more than 15° C./s , the transformation from cementite formed during the heating to austenite is accelerated, and accordingly austenite tends to become lumpy during the subsequent soaking, leading to a coarse secondary phase in final microstructure. Therefore, the average heating rate up to soaking temperature is 15° C./s or less. The average heating rate is preferably 10° C./s or less. The average heating rate is more preferably 8° C./s or less. On the other hand, although the lower limit of the average heating rate is not particularly limited, the rate is preferably 1° C./s or more, more preferably 2° C./s or more and still more preferably 3° C./s or more from the perspective of production efficiency and the like.

Soaking Temperature: 740° C. or Higher and 860° C. or Lower

When the soaking temperature is lower than 740° C., the volume fraction of ferrite increases during the subsequent temperature holding, and the area ratio of ferrite in the eventually obtained microstructure becomes too large. Excessive ferrite renders it difficult to secure a TS of 980 MPa or more for a high-strength thin steel sheet. Therefore, the soaking temperature is 740° C. or higher. The soaking temperature is preferably 750° C. or higher. The soaking temperature is more preferably 760° C. or higher. The soaking temperature is still more preferably 770° C. or higher. On the other hand, when the soaking temperature exceeds 860° C., the secondary phase tends to become coarse and lumpy when the steel sheet is held at the soaking temperature, and accordingly the secondary phase in final microstructure becomes coarse and lumpy and ductility deteriorates. Therefore, the soaking temperature is 860° C. or lower. The soaking temperature is preferably 850° C. or lower. The soaking temperature is more preferably 840° C. or lower. The soaking temperature is still more preferably 830° C. or lower.

Soaking Time: 60 Seconds or More and 600 Seconds or Less

Then, the steel sheet that has been heated to the soaking temperature is held at the soaking temperature for 60 seconds or more and 600 seconds or less. The time of being held at the soaking temperature is hereinafter referred to as “soaking time”. When the soaking time is less than 60 seconds, ductility of the eventually obtained high-strength thin steel sheet deteriorates. This is because when the soaking time is less than 60 seconds, C and Mn, which are austenite stabilizing elements, can not sufficiently concentrate into austenite during the soaking. Insufficient concentration of C and Mn in retained austenite of final microstructure lowers stability of the retained austenite, leading to deterioration in ductility. Therefore, the soaking time is 60 seconds or more. The soaking time is preferably 100 seconds or more. On the other hand, when the soaking time exceeds 600 seconds, the secondary phase tends to become coarse and lumpy during the soaking, and accordingly the secondary phase in final microstructure becomes coarse and lumpy. As a result, ductility deteriorates. Therefore, the soaking time is 600 seconds or less. The soaking time is preferably 500 seconds or less. The soaking time is more preferably 400 seconds or less. The soaking time is still more preferably 300 seconds or less.

Average Cooling Rate Down to Cooling End Temperature: 50° C./s or Less

When the soaking time has elapsed, the steel sheet is cooled down to a cooling end temperature which will be described later. The average cooling rate from the soaking temperature to cooling end temperature (hereinafter “average cooling rate down to cooling end temperature”) is 50° C./s or less. When the average cooling rate down to cooling end temperature exceeds 50° C./s, ferrite formation during the cooling is suppressed and accordingly austenite coarsens when the cooling ends. As a result, the secondary phase in final microstructure also coarsens, leading to deterioration in ductility. Therefore, the average cooling rate is 50° C./s or less. The average cooling rate is preferably 40° C./s or less. The average cooling rate is more preferably 30° C./s or less. On the other hand, although the lower limit of the average cooling rate is not particularly limited, the rate is preferably 5° C./s or more and more preferably 10° C./s or more from the perspective of production efficiency and the like.

For the cooling, any cooling method can be used alone or in combination. Examples of the cooling method include gas cooling, furnace cooling, mist cooling, roll-chilling, and water cooling.

Cooling End Temperature: 350° C. or Higher and 550° C. or Lower

When the cooling end temperature exceeds 550° C., formation of retained austenite is suppressed, and accordingly ductility deteriorates. Therefore, the cooling end temperature is 550° C. or lower. The cooling end temperature is preferably 520° C. or lower. The cooling end temperature is more preferably 490° C. or lower. On the other hand, when the cooling end temperature is less than 350° C., martensite excessively forms, and accordingly strength of the eventually obtained steel sheet becomes too high to ensure ductility. Therefore, the cooling end temperature is 350° C. or higher. The cooling end temperature is preferably 380° C. or higher. The cooling end temperature is more preferably 400° C. or higher.

Holding Time: 30 Seconds or More and 1200 Seconds or Less

When the cooling has ended, the steel sheet is held at the cooling end temperature for 30 seconds or more and 1200 seconds or less. Hereinafter, the time of being held at the cooling end temperature is referred to as “holding time”. When the holding time is less than 30 seconds, a desired amount of retained austenite is difficult to be obtained and martensite is excessively formed. As a result, ductility deteriorates. Therefore, the holding time is 30 seconds or more. The holding time is preferably 50 seconds or more. The holding time is more preferably 100 seconds or more. The holding time is still more preferably 200 seconds or more. On the other hand, even if the holding time exceeds 1200 seconds, ductility cannot be significantly improved since the producing amount of retained austenite does not further increase. On the contrary, productivity is lowered. Therefore, the holding time is 1200 seconds or less. The holding time is preferably 800 seconds or less. The holding time is more preferably 500 seconds or less.

[Zinc or Zinc Alloy Coating Treatment]

Furthermore, the steel sheet that has been held at the cooling end temperature can be subjected to zinc or zinc alloy coating treatment. The “zinc or zinc alloy coating treatment” here includes both of galvanizing treatment and zinc alloy coating treatment. The zinc alloy coating treatment can be a treatment for coating any zinc-contained alloy. In the zinc alloy coating treatment, it is preferable to form a zinc alloy coated layer such as a 5% Al-Zn alloy coated layer containing 5 mass % of Al or a 55% Al-Zn alloy coated layer containing 55 mass % of Al.

The zinc or zinc alloy coating treatment can be performed by any method, yet it is preferably hot-dip zinc or zinc alloy coating treatment. The hot-dip zinc or zinc alloy coating treatment can be alloyed hot-dip zinc or zinc alloy coating treatment. The alloyed hot-dip zinc or zinc alloy coating treatment is preferably in a way of hot-dip zinc or zinc alloy coating treatment followed by alloying treatment in a temperature range of 450° C. or higher and 550° C. or lower. Coating conditions of the zinc or zinc alloy coating treatment such as coating amount and coating device to be used are arbitrary.

EXAMPLES

The operation and effect of the high-strength thin steel sheet and the production method according to the present disclosure will be described below based on examples.

Starting steel sheets were prepared and then subjected to heat treatment to obtain high-strength thin steel sheets. The specific procedure was as follows.

Preparing Starting Steel Sheets

Hot rolled steel sheets and cold rolled steel sheets to be used as the starting steel sheets were prepared under various conditions. First, vacuum melting steel having the chemical composition listed in Table 1 was smelted in a laboratory to produce sheet-bar slabs with a thickness of 20 mm. The obtained sheet-bar slabs were heated to 1250° C. and then hot rolled at a finisher delivery temperature of 900° C. to obtain hot rolled steel sheets with a thickness of 2.4 mm. After the hot rolling, a coiling-equivalent heat treatment which simulated a coiling process at the coiling temperature listed in Tables 2and 3 was performed. Specifically, in the coiling-equivalent heat treatment, the steel sheet that had been hot rolled was cooled down to the coiling temperature listed in Tables 2 and 3 at an average cooling rate of 40° C./s , then held at the coiling temperature listed in Tables 2 and 3 for one hour, and then furnace cooled. In a case where neither cold rolling nor heat treatment was required, a hot rolled steel sheet to be used as the starting steel sheet was obtained by the above procedure. In a case where heat treatment was required but cold rolling was not required, for example No. 2, the starting steel sheet was obtained by performing a heat treatment under the conditions of the heat treatment following rolling listed in Table 2 after the coiling process.

In a case where cold rolling is required as well, the resulting hot rolled steel sheet of the above procedure was pickled with hydrochloric acid and then cold rolled at a rolling reduction of 50% to obtain a cold rolled steel sheet with a thickness of 1.2 mm. Then, the obtained cold rolled steel sheet was subjected to a heat treatment under the conditions of the heat treatment following rolling listed in Tables 2 and 3.

The microstructure of the starting steel sheet obtained as described above, in other words the starting microstructure was measured by a method which will be described later. The measurement results are listed in Tables 2 and 3.

Subsequently, the steel sheets were subjected to heating, soaking, cooling, and a holding process when the cooling had ended under the heat treatment conditions listed in Tables 2 and 3 to produce high-strength thin steel sheets. Note that Nos. 18 and 19 were cooled down to room temperature during the heat treatment following cold rolling. Additionally, some of the high-strength thin steel sheets were further subjected to hot-dip galvanizing treatment to obtain high-strength hot-dip galvanized thin steel sheets. In the hot-dip galvanizing treatment, the steel sheet was immersed in a galvanizing bath at 475° C. containing 0.13 mass % of Al for 3 seconds. The obtained galvanized layer had a coating weight of 45 g/m² per side. Some of the high-strength hot-dip galvanized thin steel sheets were further subjected to alloying treatment to obtain high-strength galvannealed thin steel sheets.

TABLE 1 Steel sample Chemical composition (mass %)* ID C Si Mn P S Al N Ti Nb V Cr Ni A 0.24 1.44 1.86 0.005 0.0010 0.029 0.0033 — — — — — B 0.22 1.58 2.63 0.004 0.0008 0.026 0.0029 — — — — — C 0.18 1.38 2.15 0.008 0.0016 0.030 0.0032 0.022 — — — — D 0.24 0.95 2.16 0.016 0.0017 0.034 0.0028 — 0.021 — — — E 0.17 1.32 1.88 0.016 0.0024 0.028 0.0035 0.015 — 0.052 — — F 0.30 1.25 2.45 0.010 0.0017 0.029 0.0031 — — — 0.18 — G 0.26 1.34 1.85 0.013 0.0021 0.035 0.0032 — — — — 0.21 H 0.21 1.48 2.17 0.008 0.0027 0.033 0.0031 — — — — — I 0.09 1.32 1.51 0.013 0.0019 0.035 0.0036 — — — — — J 0.38 1.47 2.56 0.009 0.0018 0.034 0.0029 — — — — — K 0.24 0.36 1.77 0.008 0.0025 0.037 0.0032 — — — — — L 0.21 1.24 1.28 0.021 0.0022 0.032 0.0030 — — — — — M 0.25 1.36 3.19 0.013 0.0017 0.029 0.0031 — — — — — N 0.11 1.42 3.20 0.003 0.0008 0.033 0.0042 — — — — — O 0.33 1.36 1.53 0.011 0.0029 0.042 0.0035 — — — — — Steel sample Chemical composition (mass %)* Ac₃ ID Mo Cu B Ca REM C/Mn (° C.) Remarks A — — — — — 0.13 833 Conforming steel B — — — — — 0.08 819 Conforming steel C — — 0.0015 — — 0.08 844 Conforming steel D — — — — — 0.11 804 Conforming steel E — — — — — 0.09 854 Conforming steel F 0.12 — — — — 0.12 801 Conforming steel G — 0.17 — — — 0.14 821 Conforming steel H — — — 0.0005 0.0011 0.10 834 Conforming steel I — — — — — 0.06 879 Comparative steel J — — — — — 0.15 790 Comparative steel K — — — — — 0.14 791 Comparative steel L — — — — — 0.16 849 Comparative steel M — — — — — 0.08 787 Comparative steel N — — — — — 0.03 826 Comparative steel O — — — — — 0.22 828 Comparative steel *The balance is Fe and incidental impurities. Underline indicates that it is outside the range of the present disclosure.

TABLE 2 Conditions of the heat treatment following rolling Cooling Soaking Cooling Holding Starting structure*² Steel Coiling Soaking Soaking Average end Holding Area ratio (%) sample temperature*¹ Cold temperature time cooling rate temperature time BF + Balance No. ID (° C.) rolling (° C.) (second) (° C./s) (° C.) (second) BF RA RA M structure 1 A 650 Performed 900 100 10 400 300 72 17 89 6 F 2 A 650 — 900 100 10 400 300 72 17 89 6 F 3 A 650 Performed 900 100 10 400 300 72 17 89 6 F 4 B 650 Performed 900 100 10 400 300 68 14 82 14 F 5 C 650 Performed 900 100 10 400 300 66 12 78 13 F 6 D 650 Performed 900 100 10 400 300 74 10 84 11 F 7 E 650 Performed 900 100 10 400 300 62 10 72 11 F 8 F 650 Performed 900 100 10 400 300 59 14 73 14 F 9 G 450 — — — — — — 66 12 78 13 F 10 H 650 Performed 900 100 10 400 300 64 10 74 12 F 11 I 650 Performed 900 100 10 400 300 68 5 73 13 F 12 J 650 Performed 900 100 10 400 300 63 13 76 12 F 13 K 650 Performed 900 100 10 400 300 72 3 75 14 F 14 L 650 Performed 900 100 10 400 300 70 11 81 14 F 15 M 650 Performed 900 100 10 400 300 64 14 78 17 F 16 N 650 Performed 900 100 10 400 300 66 5 71 20 F 17 A 650 Performed 800 100 10 400 300 43 8 51 15 F 18 A 650 Performed 800 100 100 —*⁶ 0 0  0 48 F 19 A 650 Performed 900 100 100 —*⁶ 0 0  0 100 — 20 A 650 Performed 900 100 10 650 300 0 0  0 0 F, P 21 I 650 Performed 800 100 10 400 300 35 4 39 10 F 22 C 650 Performed 900 100 10 400 300 72 12 84 10 F 23 C 650 Performed 900 100 10 400 300 72 12 84 10 F 24 C 650 Performed 900 100 10 400 300 72 12 84 10 F Heat treatment conditions Heating Cooling First Second Soaking Cooling Holding heating heating Soaking Soaking Average end Holding rate*³ rate*⁴ temperature time cooling rate temperature time Coating No. (° C./s) (° C./s) (° C.) (second) (° C./s) (° C.) (second) treatment*⁵ Remarks  1 20 5 805 100 20 450 300 — Example  2 20 5 795 100 20 450 300 GA Example  3 20 5 800 100 20 450 300 GI Example  4 20 5 780 100 20 450 300 — Example  5 20 5 800 100 20 450 300 — Example  6 20 5 770 100 20 450 300 — Example  7 20 5 805 100 20 450 300 — Example  8 20 5 770 100 20 450 300 — Example  9 20 5 785 100 20 450 300 — Example 10 20 5 795 100 20 450 300 — Example 11 20 5 830 100 20 450 300 — Comparative Example 12 20 5 780 100 20 450 300 — Comparative Example 13 20 5 760 100 20 450 300 — Comparative Example 14 20 5 810 100 20 450 300 — Comparative Example 15 20 5 760 100 20 450 300 — Comparative Example 16 20 5 780 100 20 450 300 — Comparative Example 17 20 5 800 100 20 450 300 — Comparative Example 18 20 5 790 100 20 450 300 — Comparative Example 19 20 5 795 100 20 450 300 — Comparative Example 20 20 5 795 100 20 450 300 — Comparative Example 21 20 5 795 100 20 450 300 — Comparative Example 22  5 5 805 100 20 450 300 — Comparative Example 23 20 20  790 100 20 450 300 — Comparative Example 24 20 — 700 100 20 450 300 — Comparative Example *¹Coiling temperature after hot rolling during the manufacturing of starting steel sheet *²BF: bainitic ferrite, RA: retained austenite, M: martensite, F: ferrite, P: perlite *³Average heating rate up to 700° C. *⁴Average heating rate from 700° C. up to soaking temperature *⁵GA: galvannealing, GI: hot-dip galvanizing *⁶Cooled to room temperature Underline indicates that it is outside the range of the present disclosure.

TABLE 3 Conditions of the heat treatment following rolling Cooling Soaking Cooling Holding Starting structure*² Steel Coiling Soaking Soaking Average end Holding Area ratio (%) sample temperature*¹ Cold temperature time cooling rate temperature time BF + Balance No. ID (° C.) rolling (° C.) (second) (° C./s) (° C.) (second) BF RA RA M structure 25 C 650 Performed 900 100 10 400 300 72 12 84 10 F 26 C 650 Performed 900 100 10 400 300 72 12 84 10 F 27 F 650 Performed 900 100 10 400 300 64 13 77 13 F 28 F 650 Performed 900 100 10 400 300 64 13 77 13 F 29 F 650 Performed 900 100 10 400 300 64 13 77 13 F 30 F 650 Performed 900 100 10 400 300 64 13 77 13 F 31 O 600 Performed 900 100 10 400 300 63 19 82 14 F 32 A 410 — — — — — — 76 11 87 7 F 33 B 480 — — — — — — 77 13 90 5 F 34 C 530 — — — — — — 72 10 82 9 F, P 35 E 370 — — — — — — 81 8 89 6 F 36 A 650 — — — — — — 0 0  0 0 F, P 37 A 600 Performed 860 100 15 420 400 72 16 88 7 F 38 A 600 Performed 860 100 15 420 400 73 18 91 6 F 39 A 620 Performed 860 200 30 360 1200 75 8 83 8 F 40 A 640 Performed 860 200 30 360 1200 76 6 82 5 F 41 A 620 Performed 860 200 30 360 1200 79 7 86 4 F 42 A 600 Performed 860 200 30 360 1200 77 8 85 5 F 43 B 700 Performed 840 550 45 380 800 69 10 79 10 F 44 B 680 Performed 840 550 45 380 800 73 9 82 8 F 45 B 680 Performed 840 550 45 380 800 71 10 81 9 F 46 B 670 Performed 840 550 45 380 800 72 8 80 8 F 47 C 660 Performed 920 60 5 520 45 65 14 79 13 F 48 C 650 Performed 920 60 5 520 45 66 13 79 12 F 49 C 660 Performed 920 60 5 540 45 68 13 81 13 F, P 50 C 640 Performed 920 60 5 540 45 64 14 78 15 F, P Heat treatment conditions Heating Cooling First Second Soaking Cooling Holding heating heating Soaking Soaking Average end Holding rate*³ rate*⁴ temperature time cooling rate temperature time Coating No. (° C./s) (° C./s) (° C.) (second) (° C./s) (° C.) (second) treatment*⁵ Remarks 25 20 5 900 100 20 450 300 — Comparative Example 26 20 5 805  10 20 450 300 — Comparative Example 27 20 5 780 100 100  450 300 — Comparative Example 28 20 5 785 100 20 300 300 — Comparative Example 29 20 5 780 100 20 650 300 — Comparative Example 30 20 5 775 100 20 450  15 — Comparative Example 31 20 5 800 100 20 450 300 — Comparative Example 32 20 5 820 120 20 400 500 — Example 33 25 10 780  80 30 400 300 — Example 34 20 5 800 150 20 400 200 — Example 35 30 5 850 500 15 400  50 — Example 36 20 5 800 100 20 400 250 — Comparative Example 37 15 5 790 100 30 400 200 — Example 38 30 15 790 100 30 400 200 — Example 39 40 10 720 180 15 480 400 — Comparative Example 40 40 10 750 180 15 480 400 — Example 41 40 10 870 180 15 480 400 — Comparative Example 42 40 10 850 180 15 480 400 — Example 43 20 5 810  60 20 380 600 — Example 44 20 5 810 700 20 380 600 — Comparative Example 45 20 5 810 550 20 380 600 — Example 46 20 5 810 300 45 380 600 — Example 47 30 5 780 150 30 350 300 — Example 48 30 5 780 150 30 540 300 — Example 49 30 5 780 150 30 420  30 — Example 50 30 5 780 150 30 420 1200  — Example *¹Coiling temperature after hot rolling during the manufacturing of starting steel sheet *²BF: bainitic ferrite, RA: retained austenite, M: martensite, F: ferrite, P: perlite *³Average heating rate up to 700° C. *⁴Average heating rate from 700° C. up to soaking temperature *⁵GA: galvannealing, GI: hot-dip galvanizing Underline indicates that it is outside the range of the present disclosure.

Evaluation of Microstructure and Tensile Property

The microstructure and tensile property of each high-strength thin steel sheet obtained as described above were evaluated by the following method. The obtained results are listed in Tables 4 and 5. The microstructure of the starting steel sheet listed in Tables 2 and 3 was also evaluated by the same method.

(Area Ratio of Retained Austenite)

The area ratio of retained austenite was measured through X-ray diffraction. In the measurement, the steel sheet surface was polished to a depth of one-fourth of the sheet thickness to expose a plane of sheet thickness ¼, and the x-ray diffraction intensity of the plane of sheet thickness ¼ was determined. Specifically, the volume ratio of retained austenite was calculated from the peak intensity ratio of (200) and (211) planes of BCC phase and (200), (220) and (311) planes of FCC phase through X-ray diffraction using Co K-alpha X-ray. The measurement position was at the center of the width direction of the steel sheet. The microstructure of the steel sheet of the present disclosure is three-dimensionally uniform, and therefore the volume ratio can be regarded as the area ratio of retained austenite. Measurement was carried out at three positions randomly selected on one high-strength thin steel sheet, and the average value of the obtained values was taken as the area ratio of retained austenite.

(Area Ratio of Microstructure Other than Retained Austenite)

The area ratio of each microstructure other than retained austenite was obtained by etching a cross section in the rolling direction of the high-strength thin steel sheet with nital and then observing the cross section with a scanning electron microscope (SEM). The observation position was at the center of the width direction of the steel sheet and at a depth of one-fourth of the sheet thickness. An SEM image of the observation position was photographed at a magnification of 2000 times, and the area ratio of each microstructure was calculated through image analysis of the obtained SEM image. In the image analysis, the occupied area of each microstructure present in a randomly set 50μm×50 μm square region was determined and the area ratio thereof was calculated. The observation was carried out at five positions randomly selected on one high-strength thin steel sheet, and the average value of the obtained values was taken as the area ratio.

Each microstructure in the image analysis was identified by the following method.

Retained austenite and martensite are observed as massive white regions having a relatively smooth surface in the SEM image. Therefore, the area ratio of martensite was calculated by subtracting the area ratio of retained austenite measured by the above-mentioned method from the area ratio of the white region.

On the other hand, ferrite, tempered bainitic ferrite and bainitic ferrite are observed as black regions in the SEM image. Among the black regions, those in a massive form were identified as ferrite. Additionally, among the black regions, those having an elongated shape and a recognizable substructure were identified as bainitic ferrite, and those in which substructure could not be recognized were identified as tempered bainitic ferrite. Examples of the substructure include internal structure of lath, block, packet, etc. in grains.

In the above measurement, the grain size to be measured had a lower limit of 0.01 μm which is the measurement limit.

(Secondary Phase)

The average equivalent circular diameter of secondary phase, the area ratio of coarse grain in secondary phase and the average minor axis length of secondary phase were obtained by etching a cross section in the rolling direction of the high-strength thin steel sheet with nital and then observing the cross section with a scanning electron microscope (SEM). The observation position was at the center of the width direction of the steel sheet and at a depth of one-fourth of the sheet thickness. An SEM image of the observation position was photographed at a magnification of 2000 times, and the obtained SEM image was subjected to image analysis. In the image analysis, occupied area and minor axis length of the secondary phase present in a randomly set 50 μm×50 μm square region were determined. The average equivalent circular diameter of secondary phase was calculated from the obtained occupied area of the secondary phase. The measurement lower limit of the minor axis length was 0.01 μm. The observation was carried out at five positions randomly selected on one high-strength thin steel sheet, and the average value of the obtained values was adopted.

(Tensile Property)

A No. 5 test piece as specified in JIS Z 2201 having a longitudinal direction, which is also the tensile direction, in a direction at 90 degrees from the rolling direction was taken from the high-strength thin steel sheet. A tensile test was carried out according to JIS Z 2241 using the obtained test piece to measure the tensile strength (TS), yield point (YP) and elongation (El).

Additionally, the strength-elongation balance represented by the product of tensile strength (TS) and elongation (El) and expressed as TS×El was calculated from the obtained values and the tensile property was evaluated based on the value of TS×El. The evaluation criteria were as follows.

Excellent: 25000 MPa·%≤TS×El

Good: 22000 MPa·%≤TS×El<25000 MPa·%

Poor: TS×El<22000 MPa·%

(Elongation Anisotropy)

The following measurement was carried out to evaluate the elongation anisotropy. First, a tensile test was carried out taking the rolling direction, a direction at 45 degrees from the rolling direction, and a direction at 90 degrees from the rolling direction respectively as the tensile direction to measure the El of each direction. Except that the tensile direction was set as described above, the tensile test was carried out in the same manner as the one for evaluating the tensile property. Then, ΔEl, which is defined by the following equation, was calculated with the maximum value (max.El) and the minimum value (min.El) of the obtained El values of the three directions.

ΔEl=(max·El)−(min·El)

The ΔEl is preferably 3% or less and more preferably 2% or less from the perspective of formability. On the other hand, since the smaller the ΔEl is, the better, the lower limit can be 0%. Nevertheless, an ΔEl of 0.5% or more or 1.0% or more is also acceptable.

As can be seen from the results listed in Tables 4 and 5, the high-strength thin steel sheets satisfying the requirements of the present disclosure had a TS of 980 MPa or more, a TS×El of 22000 MPa·% or more and an ΔEl of 3% or less. In contrast, the steel sheets not satisfying the requirements of the present disclosure had a TS×El of less than 22000 MPa·%, and some of them also had an ΔEl exceeding 3%. Thus, according to the present disclosure, a high-strength thin steel sheet excellent in both tensile strength and elongation with small elongation anisotropy can be obtained.

TABLE 4 Microstructure of high-strength thin steel sheet Secondary phase*² Steel Average equivalent Area ratio of Average sample Area ratio (%)*¹ circular diameter coarse grain*³ minor axis No. ID F + TBF + BF M RA (μm) (%) (μm) 1 A 64 19 17 1.1 5 0.26 2 A 60 19 15 1.5 3 0.34 3 A 63 18 19 1.4 4 0.22 4 B 57 25 18 1.8 9 0.39 5 C 59 20 21 1.2 8 0.37 6 D 65 18 17 1.1 6 0.35 7 E 58 23 19 1.5 4 0.33 8 F 56 26 18 1.2 5 0.38 9 G 58 22 20 1.9 8 0.24 10 H 61 23 16 1.8 4 0.32 11 I 82 14  4 1.6 6 0.22 12 J 48 38 14 1.6 5 0.39 13 K 68 29  3 1.7 8 0.34 14 L 74 19  7 1.6 6 0.25 15 M 55 38  7 1.8 9 0.38 16 N 52 44  4 1.9 9 0.36 17 A 64 26 10 2.6 6 0.32 18 A 62 35  3 3.1 13  0.53 19 A 65 29  6 1.5 7 0.33 20 A 67 25  8 2.5 12  0.67 21 I 75 18  7 1.2 7 0.21 22 C 60 28 12 3.5 18  0.86 23 C 62 25 13 4.2 17  0.67 24 C 73 15 12 1.7 5 0.21 Evaluation results TS YP El TS × El ΔEl No. (MPa) (MPa) (%) (MPa · %) Evaluation (%) Remarks  1 1028 852 28.3 29092 Excellent 1.7 Example  2 1045 646 26.5 27693 Excellent 1.8 Example  3 996 640 27.9 27788 Excellent 1.5 Example  4 1251 993 18.7 23394 Good 2.7 Example  5 1082 694 24.4 26401 Excellent 1.6 Example  6 1158 931 19.3 22349 Good 1.8 Example  7 1055 666 21.5 22683 Good 2.4 Example  8 1281 844 18.3 23442 Good 1.6 Example  9 1041 653 26.0 27066 Excellent 1.7 Example 10 1012 716 28.2 28538 Excellent 1.9 Example 11 792 547 20.5 16236 Poor 2.6 Comparative Example 12 1384 944 11.5 15916 Poor 2.7 Comparative Example 13 1256 961 15.8 19845 Poor 2.1 Comparative Example 14 917 625 18.2 16689 Poor 2.3 Comparative Example 15 1369 1114 13.1 17934 Poor 2.2 Comparative Example 16 1372 878 12.3 16876 Poor 2.6 Comparative Example 17 995 674 17.2 17114 Poor 2.8 Comparative Example 18 1132 924 14.9 16867 Poor 3.5 Comparative Example 19 1087 772 16.5 17936 Poor 2.3 Comparative Example 20 1035 828 16.7 17285 Poor 3.4 Comparative Example 21 891 589 22.8 20315 Poor 3.2 Comparative Example 22 1265 987 13.6 17204 Poor 1.6 Comparative Example 23 1078 657 15.8 17032 Poor 2.8 Comparative Example 24 854 619 20.2 17251 Poor 2 Comparative Example *¹F: ferrite, TBF: tempered bainitic ferrite, BF: bainitic ferrite, M: martensite, RA: retained austenite *²martensite and retained austenite *³area ratio of martensite and retained austenite grains having an equivalent circular diameter of 2.0 μm or more to the whole secondary phase Underline indicates that it is outside the range of the present disclosure.

TABLE 5 Microstructure of high-strength thin steel sheet Secondary phase*² Steel Average equivalent Area ratio of Average sample Area ratio (%)*¹ circular diameter coarse grain*³ minor axis No. ID F + TBF + BF M RA (μm) (%) (μm) 25 C 50 28 22 3.8 12  0.73 26 C 68 25  7 1.9 7 0.36 27 F 38 29 33 1.8 8 0.38 28 F 43 45 12 3.6 21  0.84 29 F 69 29  2 1.3 8 0.25 30 F 52 42  6 3.2 15  0.50 31 O 61 24 15 1.4 6 0.28 32 A 59 20 21 1.3 5 0.26 33 B 55 28 17 1.9 9 0.37 34 C 65 16 19 1.3 8 0.34 35 E 62 28 10 1.9 10  0.38 36 A 61 31  8 3.6 32  1.32 37 A 63 25 12 2.0 9 0.38 38 A 63 22 15 1.9 8 0.36 39 A 76 13 11 1.1 5 0.27 40 A 67 18 15 1.2 5 0.28 41 A 39 29 32 3.4 19  0.75 42 A 50 26 24 1.8 9 0.37 43 B 67 20 13 1.5 6 0.29 44 B 39 29 32 1.4 5 0.23 45 B 49 23 28 1.3 5 0.28 46 B 54 22 24 1.5 6 0.24 47 C 52 27 21 1.9 8 0.21 48 C 68 21 11 1.4 5 0.18 49 C 59 28 13 1.6 6 0.21 50 C 57 19 24 1.3 4 0.19 Evaluation results TS YP El TS × El ΔEl No. (MPa) (MPa) (%) (MPa · %) Evaluation (%) Remarks 25 1056 828 12.8 13517 Poor 3.1 Comparative Example 26 982 601 16.7 16399 Poor 2.6 Comparative Example 27 1221 767 12.5 15263 Poor 2.3 Comparative Example 28 1356 946 10.3 13967 Poor 2.4 Comparative Example 29 965 658 18.6 17949 Poor 2.8 Comparative Example 30 1284 810 13.2 16949 Poor 2.6 Comparative Example 31 1357 915 12.6 17098 Poor 2.5 Comparative Example 32 1036 724 27.4 28386 Excellent 1.6 Example 33 1205 921 18.8 22654 Good 2.6 Example 34 1109 706 25.6 28390 Excellent 1.3 Example 35 1132 727 19.5 22074 Good 2.3 Example 36 1158 784 11.6 13433 Poor 1.7 Comparative Example 37 1081 908 21.1 22809 Good 2.4 Example 38 1056 819 22.6 23866 Good 2.1 Example 39 918 697 23.6 21665 Poor 2.8 Comparative Example 40 993 706 23.4 23236 Good 2.7 Example 41 1237 948 12.8 15834 Poor 1.6 Comparative Example 42 1159 894 19.5 22601 Good 1.9 Example 43 1269 1023 18.1 22969 Good 1.6 Example 44 1312 1064 15.6 20467 Poor 1.5 Comparative Example 45 1238 995 18.6 23027 Good 1.6 Example 46 1206 968 20.1 24241 Good 1.8 Example 47 1121 755 20.3 22756 Good 1.9 Example 48 1065 648 21.6 23004 Good 2.4 Example 49 1158 766 19.8 22928 Good 2.6 Example 50 1037 682 24.8 25718 Excellent 2.7 Example *¹F: ferrite, TBF: tempered bainitic ferrite, BF: bainitic ferrite, M: martensite, RA: retained austenite *²martensite and retained austenite *³area ratio of martensite and retained austenite grains having an equivalent circular diameter of 2.0 μm or more to the whole secondary phase Underline indicates that it is outside the range of the present disclosure. 

1-7. (canceled)
 8. A high-strength thin steel sheet comprising: a chemical composition containing, by mass %, C: 0.10% or more and 0.35% or less, Si: 0.5% or more and 2.0% or less, Mn: 1.5% or more and 3.0% or less, P: 0.001% or more and 0.050% or less, S: 0.0001% or more and 0.0100% or less, Al: 0.001% or more and 1.00% or less, and N: 0.0005% or more and 0.0200% or less, the balance being Fe and inevitable impurities, and a ratio of C content in mass % to Mn content in mass %, expressed as C/Mn, being 0.08 or more and 0.20 or less; and a microstructure where a total area ratio of ferrite, tempered bainitic ferrite and bainitic ferrite is 40% or more and 70% or less, an area ratio of martensite is 5% or more and 30% or less, and an area ratio of retained austenite is 10% or more and 35% or less, to a total area of the microstructure of the high-strength thin steel sheet, an average equivalent circular diameter of martensite and retained austenite grains is 2.0 μm or less, an area ratio of martensite and retained austenite grains having an equivalent circular diameter of 2.0 μm or more to a total area of martensite and retained austenite is 10% or less, and an average minor axis length of martensite and retained austenite grains is 0.40 μm or less.
 9. The high-strength thin steel sheet according to claim 8, wherein the chemical composition further contains one or more selected from among group (i) to group (iii) below: group (i): at least one selected from the group consisting of Ti: 0.005% or more and 0.100% or less, Nb: 0.005% or more and 0.100% or less, and V: 0.005% or more and 0.100% or less, in mass %; group (ii): at least one selected from the group consisting of Cr: 0.05% or more and 1.0% or less, Ni: 0.05% or more and 0.50% or less, Mo: 0.05% or more and 1.0% or less, Cu: 0.005% or more and 0.500% or less, and B: 0.0001% or more and 0.0100% or less, in mass %; and group (iii): at least one of Ca: 0.0001% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less, in mass %.
 10. The high-strength thin steel sheet according to claim 8, further comprising a zinc or zinc alloy coated layer on surface.
 11. The high-strength thin steel sheet according to claim 9, further comprising a zinc or zinc alloy coated layer on surface.
 12. A method of producing a high-strength thin steel sheet, comprising preparing a steel sheet having the chemical composition according to claim 8 and a microstructure where a total area ratio of bainitic ferrite and retained austenite to a total area of the microstructure of the steel sheet is 70% or more, heating the steel sheet to 700° C. at an average heating rate of 15° C./s or more, heating the steel sheet from 700° C. to a soaking temperature of 740° C. or higher and 860° C. or lower at an average heating rate of 15° C./s or less, holding the heated steel sheet at the soaking temperature for 60 seconds or more and 600 seconds or less, cooling the steel sheet to a cooling end temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 50° C./s or less, and holding the cooled steel sheet at the cooling end temperature for 30 seconds or more and 1200 seconds or less.
 13. A method of producing a high-strength thin steel sheet, comprising preparing a steel sheet having the chemical composition according to claim 9 and a microstructure where a total area ratio of bainitic ferrite and retained austenite to a total area of the microstructure of the steel sheet is 70% or more, heating the steel sheet to 700° C. at an average heating rate of 15° C./s or more, heating the steel sheet from 700° C. to a soaking temperature of 740° C. or higher and 860° C. or lower at an average heating rate of 15° C./s or less, holding the heated steel sheet at the soaking temperature for 60 seconds or more and 600 seconds or less, cooling the steel sheet to a cooling end temperature of 350° C. or higher and 550° C. or lower at an average cooling rate of 50° C./s or less, and holding the cooled steel sheet at the cooling end temperature for 30 seconds or more and 1200 seconds or less.
 14. The method of producing a high-strength thin steel sheet according to claim 12, further comprising subjecting the steel sheet after being held at the cooling end temperature to zinc or zinc alloy coating treatment.
 15. The method of producing a high-strength thin steel sheet according to claim 13, further comprising subjecting the steel sheet after being held at the cooling end temperature to zinc or zinc alloy coating treatment. 